1. Field of the Invention
This invention relates to the synthesis of 1,3-difluoronaphthalene derivatives for making ferroeletric liquid crystal mixtures and their uses in ferroelectric switching and display device.
2. Description of the Related Art
In addition to nematic and cholesteric liquid crystals, recent times have also seen the use of optically active tilted smectic (ferroelectric) liquid crystals in commercial display devices.
Clark and Lagerwall showed that the use of ferroelectric liquid crystals (FLCs) in very thin cells leads to optoelectric switching or display elements having response times faster by a factor of up to 1000 than those of the conventional TN (twisted nematic) cells (see, for example, EP-A 0 032 362). On the basis of these and other favorable properties, for example the possibility of bistable switching and the contrast, which is virtually independent of viewing angle, FLCs are in principle highly suited to applications such as computer displays.
For the use of FLCs in electrooptical or completely optical assemblies there is a need either for compounds which form tilted or orthogonal smectic phases and which are themselves optically active, or else for compounds which, although forming such smectic phases are not themselves optically active, can be doped with optically active compounds to induce ferroelectric smectic phases. The desired phase should at the same time be stable over as wide as possible a temperature range.
Obtaining a good contrast ratio in electrooptical assemblies necessitates a uniform planar orientation of the liquid crystals. Good orientation in the S.sub.A and S*.sub.C phase can be achieved, for example, when the phase sequence of the liquid-crystal mixture with decreasing temperature is as follows: EQU isotropic.fwdarw.N*.fwdarw.S.sub.A .fwdarw.S*.sub.C
A precondition is that the pitch of the helix in the N* phase is very large (greater than 10 .mu.m) or, even better, is fully compensated (see, for example, T. Matsumoto et al., Proc. of the 6th Int. Display Research Conf., Japan Display, Sep. 30-Oct. 2, 1986, Tokyo, Japan, pp. 468-470; M. Murakami et al., ibid. pp. 344-pp. 347). This is done, for example, by adding one or more optically active dopes which induce, say, a right-handed helix to the chiral liquid-crystal mixture which in the N* phase has a left-handed helix, in amounts such that the helix is compensated.
For the use of the SSFLCD effect (Surface Stabilized Ferroelectric Liquid Crystal Display) of Clark and Lagerwall for uniform planar orientation a further precondition is that the pitch in the smectic C* phase is substantially greater than the thickness of the display element (Mol. Cryst. Liq. Cryst. 1983, 94, 213 and 1984, 114, 151).
The optical response time T [.mu.s] of ferroelectric liquid-crystal systems, which should be as short as possible, depends on the rotational viscosity of the system .gamma. [mPas], on the spontaneous polarization P.sub.S [nC/cm.sup.2 ] and on the electric field strength E [V/m] in accordance with the relationship ##EQU1##
Since the field strength E is determined by the electrode separation in the electrooptical component and by the applied voltage, the ferroelectric display medium must be of low viscosity and must have a high spontaneous polarization in order for a short response time to be obtained.
Finally, requirements in addition to thermal, chemical and photochemical stability are for a small optical anisotropy .DELTA.n and a small positive or, preferably, negative dielectric anisotropy .DELTA..epsilon. (see, for example, S. T. Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SID Symposium, Oct. Meeting 1985, San Diego, Calif., U.S.A.).
The entirety of these requirements can only be met with mixtures of two or more components. The basis of these mixtures (or matrix) is preferably formed by compounds which as far as possible themselves already have the desired phase sequence I.fwdarw.N.fwdarw.S.sub.A .fwdarw.S.sub.C. Further components are often added to the mixture in order to lower the melting point and to broaden the S.sub.C phase and usually the N phase as well, for inducing the optical activity, for pitch compensation and for adapting the optical and dielectric anisotropy, as far as possible without increasing the rotational viscosity, for example.
Ferroelectric liquid-crystal displays can also be operated by utilizing the DHF (Distorted Helix Formation) effect or the PSFLCD (Pitch Stabilized Ferroelectric Liquid Crystal Display, also called SBF=Short Pitch Bistable Ferroelectric) effect. The DHF effect was described by B. I. Ostrovski in Advances in Liquid Crystal Research and Applications, Oxford/Budapest, 1980, 469 ff. and the PSFLCD effect is described in DE-A 39 20 625 and EP-A 0 405 346. In contrast to the SSFLCD effect, utilizing these effects requires a liquid-crystalline material with a short S.sub.C pitch.
Naphthalene derivatives for use in liquid-crystal mixtures are known, for example, from WO-A 92/16 500. 1,2-Difluoronaphthalene derivatives are known from DE-A 195 22 167.
However, since the development--of ferroelectric liquid-crystal mixtures in particular--can in no way be regarded as complete, the manufacturers of displays are interested in a very wide variety of components for mixtures. One of the reasons for this is that only the interaction of the liquid-crystalline mixtures with the individual components of the display device or of the cells (for example the alignment layer) allows conclusions to be drawn about the quality of the liquid-crystalline mixtures too.
The object of the present invention was therefore to provide novel compounds which are suitable in liquid-crystalline mixtures for improving the profile of properties of these mixtures.